The Needs Addressed
In the past decade or two, it has become increasingly common to construct new athletic stadia and the like with enclosing domes so that such stadia can be used for desired purposes without regard to local weather conditions. One of the first such large enclosed stadia is the Astrodome in Houston, Tex. The enclosing dome of that stadium has a conventional roofing system, with local support members, placed over a complex primary supporting truss and girder network which is of conventional nature. The erection of the enclosing dome of the Astrodome required extensive use of expensive shoring erected on the stadium floor. Maintenance of that dome often requires the use of scaffolding, but in any event is periodically required and is expensive. Today, at least in some areas, the construction of a stadium dome of the kind found in the Astrodome would be prohibitively expensive.
Also, there now exist through the world many stadia and the like which are now not covered or enclosed, but which could benefit significantly by being covered or enclosed. These stadia were not originally designed to support covering roof structures. Such stadia are frequently used for sporting events and the like. If such a stadium is to be covered or enclosed, it is desirable that the enclosing roof or dome structure be light in weight and be capable of being erected rapidly without the use of shoring, the presence of which interferes with use of the stadium. It is also desirable that the loads of the dome, and of loads applied to the dome, be carried in ways which do not require major and costly modifications of the existing stadium structure.
The foregoing circumstances establish that a need exists for advanced large clear-span domes which are light in weight and structurally efficient in terms of ability to support their own weight and applied loads and in terms of carrying such loads to supporting foundations, which can be erected without the use of major shoring, which upon completion require minimum maintenance, and which are so structured that such maintenance as is required can be accomplished economically. A structurally efficient dome requires a minimum number of supporting foundations, and such foundations as are required should be simple and aesthetically pleasing. The dome structure preferably should be adaptable to existing stadia which present different perimetral configurations of spaces to be enclosed, rather than itself dictating or limiting the geometry of the space to be enclosed.
Review of the Prior Art
A few dome systems presently exist which have some, even many, but not all of these desirable characteristics; most limitations as exist in these dome systems are practical and economic in nature, rather than theoretical.
Geodesic dome systems typically require regular perimeter configurations, such as circular, square, pentagonal or hexagonal shapes, for their use. While geodesic systems sometimes include a network of supporting members across the space enclosed, they are primarily passive or stresssed-skin systems in which loads are carried and transmitted in the shell or skin members of the dome. U.S. Pat. No. 3,058,550 describes a geodesic dome system which is frameless in that it does not require a network of supporting members, such as beams, tubes or trusses, but rather carries and transmits its own and applied loads through the formed-sheet skin members and associated struts to perimeter foundations. The dome system of U.S. Pat. No. 3,058,550 is geodesic in that the principal load carrying features of the dome are arranged along geodesic lines, i.e., lines which pass over the shortest distance between two spaced points on a surface; on a sphere, geodesic lines are arcs of great circles; the science of geodesics provides a way of subdividing a sphere so as to be completely triangulated by great circles. Thus, domes according to the philosophy of U.S. Pat. No. 3,058,550 are spherically curved structures; such domes have three dimensional contour within each triangulated area of the overall dome curvature. Spherical curvature of a very large span dome causes the dome to be very high unless a very large radius of curvature is used; there is an inverse relation between radius of curvature and load carrying eficiency. More significant, however, is the fact that as geodesic domes increase in size, the thickness of the skin sheet material also increases. A point is reached where increasing size makes such a geodesic dome economically less attractive than other dome systems of comparable size. Geodesic dome systems have the advantage of being constructable without the use of shoring.
Dome systems according to U.S. Pat. No. 3,909,994 have been used advantageously to enclose spaces larger than those with which pure geodesic domes can be used economically; dome systems according to this patent are designed, fabricated and erected by the assignee of this invention under the name "Polyframe". Domes of this kind use a network of interconnected extruded aluminum structural members, and cooperating connection devices, which preferably are arranged along geodesic lines to provide triangular openings which are closed by essentially flat sheet aluminum closure elements secured around their edges to the extruded structural members and connection devices. These domes have spherical curvature with essentially smooth surfaces. Theoretically, domes of this kind can be used to span large distances, but as spans increase so do the sizes of the extruded beams increase, thus producing a practical economic limit of about 450 feet (137 meters). Large extrusions are costly, and can be made in limited places subject to long delivery delays. These circumstances effectively prevent domes of this kind from being used to enclose athletic stadia and the like where span distances on the order of 700 feet (215 meters), or greater, may be encountered.
For large spans, double dome systems have been proposed in which the structural members of the domes are arranged in concentric shells interconnected by struts, thus making the structural grid into a generally spherical truss to be suitably covered. U.S. Pat. No. 3,063,519 is a variant of this approach which uses a series of small geodesic domes according to U.S. Pat. No. 3,058,550 to enclose a locally three-dimensional network of space-spanning structural members, either inside or outside the network. Where the geodesic domes are disposed outside the network of structural members, the overall dome surface has local three-dimensional contour. In either case, the shell of multiple geodesic domes is a weather skin which is supported at spaced points by the structural network which carries its own loads and those due to the weather skin. While the hexagonal perimeter of each small geodesic dome corresponds generally in shape and size to hexagonal openings in the structural network, the dome perimeters are not secured to the network to cooperate with the network in a significant load-carrying manner. Hence, the structural network is complex and relatively heavy.
It is seen, therefore, that although existing dome systems have many advantages, they are not effectively usable where large spans are encountered and do not effectively satisfy the needs and requirements described above.